Recently, interests on energy storage technologies are more increased. As the energy storage technologies are extended to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for high-energy of a battery useable as a power source of such an electronic device increases. A lithium secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.
A lithium secondary battery developed in the early 1990's includes an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, a separator interposed between the anode and the cathode, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a non-aqueous solvent.
The non-aqueous electrolyte solution for lithium secondary batteries generally has a polarity whereas a porous polymer substrate used as a component of a separator, especially polyolefin-based film or non-woven fabric, is non-polar (hydrophobic). Therefore, it is difficult to wet separator with a non-aqueous electrolyte solution sufficiently.
If a separator in a lithium secondary battery is not sufficiently wetted with non-aqueous electrolyte solution, charging or discharging of the lithium secondary battery becomes deteriorated or disabled. That is, a lithium secondary battery can be charged or discharged only when the separator is sufficiently wet with the non-aqueous electrolyte solution.
Therefore, such poor wettability of the separator with a non-aqueous electrolyte solution has been a technical problem to be solved in the art. In order to solve the problem, low-viscous linear carbonate compounds such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate, and/or, a low-viscous linear ester compounds have been added into a non-aqueous electrolyte solution in the art.
By this, a non-aqueous solvent is adjusted to have a viscosity equal to or slightly more than 1.0 cP at 25° C. and wettability of the separator has been improved in a degree.
However, a non-aqueous solvent adjusted to have a viscosity equal to or slightly more than 1.0 cP at 25° C. may cause another problem of liquid leak. Further, the non-aqueous solvent tends to evaporate easily due to its strong volatile property. Furthermore, since a non-aqueous electrolyte solution comprising the non-aqueous solvent has a low viscosity, the non-aqueous electrolyte solution has strong flammability, which leads to safety-related problems such as firing or explosion at the time of overcharging, thermal runaway or piercing of the separator.
Thus, there is a strong demand of a non-aqueous electrolyte solution which can be used more safely in a secondary battery.
In order to meet the demand, there have been proposed methods, such as adding a surfactant or applying heat or pressure when a non-aqueous electrolyte solution is injected, but these methods need additional processes and thus they are not economical.